Clean-up include directives, forward-declare, and clean-up namespaces

[IAmNotHanni]

I am cleaning up the includes according to the following rules (the discussion is ongoing)

Overview

• If a header file is not used, do not include it. No matter if in .hpp or .cpp file
• If a header is used only in the .cpp file, forward declare it and include it only in the .cpp
• Always include the standard library headers of everything you use (no indirect includes of standard headers)
• Try to include vulkan/vulkan_core.h instead of vulkan/vulkan.h if possible
• Follow the order of includes according to our docs

Extra:

• In some cases I removed the unnecessary wrapper:: namespace if we are already in that namespace

TODO

• [x] Fix commit naming
• [x] Squash

We should add these rules to the wiki for clarity. I need your feedback.

[IceflowRE]

I would break down the rules to just those:

• Forward declare if possible
• Include all, but only the header you need for the current file
• Order all includes according to our docs